1. Field of the Invention
The present invention relates to a diffractive optical element having the diffraction grating of the concave and convex shape and an optical communication module using this diffractive optical element.
2. Description of the Related Art
In an optical communication system by an optical transmission path such as an optical fiber, a plurality of optical signals whose wavelengths are different by WDM (Wavelength Division Multiplexing) are simultaneously transmitted by the optical fiber, and for the signal sending receiving terminal, a bidirectional optical transmission module is used. In such a bidirectional optical transmission module, for the purpose that the light beam for signal sending (ascent light beam) from the light emitting element toward the terminal of the optical fiber, and the light beam for signal receiving (decent light beam) from the terminal of the optical fiber toward the light receiving element are separated, it is commonly known that the diffraction grating is used. For example, in an optical transmission module written in the U.S. Pat. No. 5,555,334 or the following Japanese patent documents 1, 2, the diffraction grating as shown in FIG. 7 is used. FIG. 7 is a perspective view showing an example of the diffraction groove formed in the diffraction grating disclosed in the U.S. Pat. No. 5,555,334, or in the Japanese patent documents 1, 2.
As shown in FIG. 7, in a diffraction grating 100, a grating groove 200 of a rectangular sectional shape is formed. When the light beam is incident on the diffraction grating 100, the diffraction grating 100 diffracts the light beam, and several diffracted light beams are generated. To the receiving light beam (descent light beam) 105 which is transmitted in the optical fiber, and projected from the optical fiber end surface, the diffraction action is given by the diffraction grating 100, and 0-order diffracted light beam 107, +1-order diffracted light beam 108, and −1-order diffracted light beam 109 are generated. In them, +1-order diffracted light beam 108 is bent to the optical axis, separated from the light beam (ascent light beam) for the signal sending, and light-converged on the light-receiving surface of the light-receiving element, and the receiving signal is detected. Hereupon, because −1-order diffracted light beam 109 is bent in the same manner, it may also be made so that the −1-order diffracted light beam 109 is light-converged on the light-receiving surface of the light-receiving element. Further, the light beam (ascent light beam) for signal sending from the light source becomes the transmission light beam (0-order diffracted light beam) which transmitted through the diffraction grating 100, and is light-converged on the end surface of the optical fiber.
In FIG. 8, each of intensity of lights of the 0-order diffracted light beam 107, +1-order diffracted light beam 108 and −1-order diffracted light beam 109 in the diffraction grating 100 in FIG. 7 is schematically shown. The intensity of light is maximum in the 0-order diffracted light beam 107, and those of +1-order diffracted light beam 108 and −1-order diffracted light beam 109 are smaller than the intensity of light of the 0-order diffracted light beam 107. Accordingly, because the receiving signal is detected on the light receiving surface of the light receiving element by using the +1-order diffracted light beam 108 (or −1-order diffracted light beam 109) whose light amount is small, the signal receiving accuracy by the receiving signal becomes poor, therefore, the module performance is lowered.
[Japanese Patent Document 1] Tokkaihei No. 07-104154
[Japanese Patent Document 2] Tokkaihei No. 07-261054